1. Field of the Invention
The invention relates to microphone pickup and electronic amplification of musical instruments, particularly acoustic musical instruments, for concerts or recordings.
2. Background Art
Pickups for electronically reproducing sound from musical instruments are of two general types, pressure and vibration. A pressure pickup or microphone has a diaphragm that vibrates in response to acoustic pressure variations in air. The diaphragm vibrations are transformed into an electrical signal. Since the human ear also has a diaphragm that works in the same way, the acoustic response of a good pressure type microphone located at an optimal distance from a musical instrument approximates the sound of the instrument in a given room. Pressure-type microphones present problems of isolation, placement, and feedback, however.
The isolation problem results from pickup sounds from both desired sources (the instrument or instruments that one wants to amplify) and undesired sources (e.g., a cough or another musical instrument that one wishes to amplify separately). The conventional approach to minimizing the isolation problem is to place the microphone close to the selected instrument to be picked up and to use a so-called directional microphone, which attempts to reject sound from unwanted directions.
Since sound radiates by the inverse square law, moving the microphone closer to the instrument reduces the isolation problem by increasing the amplitude of sound from the selected instrument relative to the sound from other sources. This solution, however, increases the placement problem. Musical instruments generate sound from different parts, such as the strings, sound box, and front and back surfaces of a violin. At a normal listening distance from a musical instrument, the characteristic sound of the instrument is an amalgam of the sounds generated from each part.
Different spots in the area close to an instrument (especially within a foot or so) yield very different sounds, most or all of which a listener would consider unnatural. When extremely close (less than a few inches), the differences become so exaggerated that one spot sounds very different from another, and it can be difficult to tell what instrument is being listened to. Also, if the instrument is not stationary, but is held by the musician (guitar, violin, flute, etc.), small normal movements of the performer produce unintended and undesired changes in dynamic level (volume) and tone quality.
Feedback is a special circumstance arising from isolation and placement problems, typically during the types of live performance where performers hear themselves by listening to monitor speakers aimed in their direction. These speakers are thus also aimed at the microphone used to pick up the sound initially. This can create a positive feedback loop that drives the speaker amplifier into saturation, producing a loud howl. The usual corrective for feedback is to use directional microphones, but this is of limited use. As a last resort, vibration pickups attached to the instruments themselves have been used. These pickups sense either the vibration of the instrument at the spot where they are attached (contact pickup) or the vibration of a metallic string (magnetic induction pickup) of a stringed instrument. As these pickups do not respond to the sound in air produced by monitor speakers or other musical instruments, feedback and isolation problems are greatly reduced. Also, because they are attached to and move with the instrument, the problems of changing volume and tone quality caused by a performer's movement are eliminated.
The drawback to using contact or induction pickups, however, is that the result is extremely low fidelity. The vibrations of a string or sounding board of a violin, for example, are drastically different from the vibrations of the air around the instrument. But what is defined as the “acoustic sound” of the instrument is what the ear hears as the vibration produced in the air in response to the sum of vibrations of all the instrument's parts, as described above. Thus, these transducers have been very effective in developing new electric instruments with their own sound (especially electric guitar and electric bass). However, their abilities are limited for the high fidelity reproduction of sound from acoustic instruments.
For the above reasons, current practice for electronically transducing and filtering live music from acoustic instruments is to use a quality directional microphone or microphones set up near, and aimed at, a single instrument or group of instruments. These microphones send their signal via a special cable to a special pre-amplifier (which sometimes sends power to the mic). This then connects to general purpose equalizer and mixing circuits. For example, in a rock band a typical drum set (five drums, one hi-hat, and two cymbals) may have one directional microphone for each drum and the hi-hat, mounted on a stand very close to the drum, plus two “overhead” directional microphones for stereo effect and to pick up the cymbals. The two overhead microphones must be at least a foot or so from the cymbals to avoid picking up a loud metallic hum. A guitar may have one or two of these mics placed between one and three feet away.
As previously mentioned, it is also common to mount pickup devices directly on individual instruments, typically guitars, to produce a different type of sound from that produced by the conventional “acoustic” form of the same kind of instrument. These pickups sense the vibration of some part of the instrument, such as the front soundboard of a guitar. Examples of such pickups are described in U.S. Pat. No. 4,051,761 of Nylen, U.S. Pat. No. 4,143,575 of Oliver, U.S. Pat. No. 4,423,654 of Yamagami, U.S. Pat. No. 4,481,854 of Dugas, U.S. Pat. No. 4,837,836 of Barcus, U.S. Pat. No. 5,136,918 of Riboloff, and U.S. Pat. No. 5,206,449 of McClish. The amplified sound from these vibration sensitive pickups mounted on either acoustic or so-called electric or electronic instruments differs intentionally from the sound produced by acoustic instruments and sensed by pressure type microphones mounted at a distance from the instrument; so these vibration pickups are not suitable for high fidelity electronic reproduction of the sound of an acoustic instrument.
It has been proposed to mount miniature pressure-sensitive microphones directly on musical instruments for specific purposes. For example, U.S. Pat. No. 4,837,836 issued to Barcus on Jun. 6, 1989 addresses the drawbacks of using stationary conventional microphones to pick up musical instruments in general, and also of holding a standard full size microphone close to, or attaching a miniature microphone directly to, an accordion or harmonica in particular. These drawbacks include feedback from nearby speakers and undesirable emphasis of the sounds coming from a localized portion of the elongated reed banks of accordions and harmonicas; that is, an increased volume of the notes whose reeds are near where the microphone is attached.
To overcome the drawbacks of the prior arrangements, Barcus provides a pickup module in which a miniature pressure-type microphone capsule is embedded. The module has an elongated narrow sound guide channel extending between oppositely facing open ends, and a sound sensitive surface area of the microphone communicates with the central region of the channel. The narrow sound channel creates a two-lobed directional sensitivity pattern for the microphone in an attempt to respond more equally to all notes when the module is centrally mounted on an elongated reed bank of a harmonica or accordion. Barcus also suggests that the module may be used with other musical instruments, and specifically that it can be attached to a drumhead near the edge of the drumhead to avoid feedback, pickup of an undesirable amount of room ambience, and lack of presence that occur with a conventional microphone stationed in front of the drum.
This lack of presence, a subjective term often used to describe a characteristic frequency band (which is different for each sound source), has not been noticed by the present inventor. However, in trials by the inventor using a variety of shapes, it has been noticed that strong, unnatural (and unpleasant) sounding frequency peaks are created by the shape of the cavity surrounding the microphone. These shaped microphone enclosures invariably add more problems than are solved in efforts to replicate the instrument's acoustic sound. The Barcus patent has a chart that looks like it may show improved frequency response, but it only shows improved evenness of volume from note to note. Test results of the inventor show each note would have a seriously degraded frequency response when compared to a high fidelity reference.
U.S. Pat. No. 5,262,586, issued to Oba et al. on Nov. 16, 1993, discloses a sound controller for an acoustic musical instrument to modify the sound produced by the instrument. In an example using a piano, the output of a detecting unit having 1) vibration sensors attached to the bridges and agraffes, 2) electromagnetic pickup units close to the strings, and 3) microphones attached to the sound board is delivered to a digital processing unit. Processors actuated by the various types of sensors for controlling loudness, delay, equalization, and phase difference deliver their output to vibration actuators mounted on the sound board and case boards of the piano. A parameter determining means adjusts the various processors so that the actuators create additional vibrations to produce acoustic sounds with modified qualities. Thus, Oba et al. use microphones as one of several types of pickups mounted inside a musical instrument to feed sounds and vibrations generated by playing the instrument to a vibration unit that alters the acoustic output of the instrument. Oba et al. do not use these microphones to produce an electronic signal in response to the acoustic output of the instrument for recording or amplifying the unmodified output. Consequently, none of the arrangements of the prior art provides a high fidelity solution to the problems of microphone isolation and placement encountered in electronic reproduction of sound from an acoustic musical instrument. Each case is currently successful only via careful tailoring by a sound engineer using sophisticated equipment.